Key to the species of Acetabularia
| 1 | Ray cell more than 12, united laterally, forming a solid disk | A. parvula |
| 1 | Ray cells less than 10, united only at their bases | 2 |
| 2 | Ray cells laterally separated, attached only at their bases in a single plane | A. clavata |
| 2 | Ray cells free, attached irregularly in many planes | A. exigua |